[TCML] High Voltage design questions and considerations

[Mike Thompson <mthompson14@xxxxxxxxx>]

Hello All,
 
I am new to the list, and was wondering if I could get some advice and/or opinions on Tesla Coil design. It’s been some time since I have played around with high voltage. About 15 years ago a friend and I built a ridiculously inefficient coil and enjoyed what fireworks it had to offer. It was really just a collection of PVC, glass, tin foil, and wire. The transformer was an oil burner igniter type. This is the only piece I actually still have.

If the transformer is of a common type, the research I did states that it has a 10KV output at 23mA. This would equate to 230 watts, meaning that the primary of the transformer should not exceed 1.9 amps of current. The transformer should also have a turn ratio of around 83:1. To prove that the transformer truly puts out 10KV I did a little testing. Below is a rough schematic of my test and some data follows the diagram.
 _____________   ________
 |     |     |||(        |
 _     _     |||(        |
|~|   |M|    )||(        -
 |     |     )||(       |M|  
 |     |     |||(        | 
 |  _  |     |||(        |
 --|M|-------|||---------|

A variac was used to alter the AC voltage going into the transformer. I used a total of three meters for this test. One to monitor the voltage of the variac, one to monitor the voltage of the transformer’s secondary, and finally one to monitor the current going from the variac into the primary of the transformer.

V In Amps In  Calc Res in Ohms V Out Calc Turns Ratio
1.3 0.0085  152.941176470588 119.3 91
2.1 0.0116  181.034482758621 187.2 89
3.2 0.0153  209.150326797386 277.8 86
4.1 0.0177  231.638418079096 346.5 84
5.2 0.0208  250   434 83
6 0.0229  262.008733624454 498 83
7.1 0.0258  275.193798449612 590 83
8.3 0.0286  290.20979020979  683 82

I took as many readings as I could until I reached the upper limit of the meter hooked up to the transformer's secondary (750 volts). Then I simply calculated the ratio of the input to the output voltage. As can be observed it appeared to be around 83:1. I am assuming that the current consumption is the result of inductive reactance or what I like to term as the "Angel's share" of the power.

I guess my questions in regards to the transformer are.  What is the maximum current that I can have going through the primary before the transformer really starts to have problems? Also why does the inductance seem to increase when I apply more voltage?

Ok enough about the transformer.
 
Now I have some questions about capacitors. Because I am on a budget I am not able to afford fancy capacitors. In fact, in the past I used patio door glass and tin foil. This tended to be bulky and dangerous in a limited space.

Recently I read about saltwater capacitors and decided to give that a try. My capacitor is simply a coffee can filed with saltwater. Inside the can I placed three water bottles containing salt water. Electrodes were inserted in each bottle and tied together. All vessels that contained water were topped off with vegetable oil.  Picture available upon request.

I used a meter to read the capacitance. The meter used 12 KHz as the frequency to test with. The reading came up to be 19.2nF. At 60Hz this would equate to be 138,155 ohms of reactance.  So at 170 Volts (The limit of my variac) I should get a reading of 1.23mA of current. This too I thought I needed to test. Below is the schematic along with the data that I logged.

 _____________
 |     |     |
 _     _     |
|~|   |M|    _
 |     |     ^
 |     |     |
 |  _  |     |
 --|M|-------|

Again a variac was used to alter the AC voltage. Two meters were put to use. One to monitor voltage and the other to monitor current. I took a reading every ten volts.

Volts Amps  Calc Xc in Ohms  Capacitance in Farads 
10 0.000072 138888.888888889 1.90989E-08 
20 0.000143 139860.13986014  1.89662E-08 
30 0.000208 144230.769230769 1.83915E-08 
40 0.00028  142857.142857143 1.85683E-08 
50 0.000349 143266.475644699 1.85153E-08 
60 0.000418 143540.669856459 1.84799E-08 
70 0.000489 143149.284253579 1.85304E-08 
80 0.000559 143112.701252236 1.85352E-08 
90 0.00063  142857.142857143 1.85683E-08 
100 0.000698 143266.475644699 1.85153E-08 
110 0.000767 143415.906127771 1.8496E-08 
120 0.000837 143369.17562724  1.8502E-08 
130 0.000909 143014.301430143 1.85479E-08 
140 0.000981 142711.518858308 1.85873E-08 
150 0.001051 142721.217887726 1.8586E-08 
160 0.001112 143884.892086331 1.84357E-08 
170 0.001192 142617.44966443  1.85995E-08 

Using Ohm’s Law, and the formula for capacitive reactance (Xc = 1/2piFC) I was able to calculate capacitance. I was pleasantly surprised to see that the values came out close to what the meter was reading at 12 KHz. Furthermore there was a general sense of satisfaction when the math actually came within the ballpark of what I was reading.

All pleasantness aside I started to have some concerns. First off, the reactance at 60Hz produced a resistance of approx 140KOhms. At 10KV the capacitor would draw 71mA. Well above the rating of 23mA of the transformer. This also explained the loud hum and 4 Amps of current being drawn by the transformer’s primary. Can the standard oil ignition transformer take this sort of punishment for very long? Next I started to wonder where this 71mA was going. Was this purely reactance?

I am aware of the dual purpose of the capacitor in a Tesla coil. It needs to be able to not draw more current then the power supply can handle at 60Hz yet be of a decent value to properly tune the coil's primary to resonate with the secondary. BUT, does the 71mA in anyway go to charging the primary of the coil? Or is it merely burned off in reactance? This also got me to thinking. If indeed the 71mA is just the "Angel’s share" and has nothing to do with charging the Tesla Coil's primary, do we then need to insure that the power supply can deal with the reactance of the capacitor, the reactance of the primary, and only then use the rest of the power to transfer to the secondary of the Tesla Coil?

Perhaps I am missing something. Is my line of thinking as least in the right direction? I have no engineering training, so to someone in the known I probably look a bit silly, but can anyone help me with these questions? As freaky as it sounds, since I have started playing with all this again I have been lying awake at night ponder these questions. Can anyone help?

Thanks
Mike T.

Homepage
http://home.comcast.net/~mikethompson236/index.htm



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